In a laser system, the harmonics of the fundamental electro-magnetic radiation of the laser source may be generated using a non-linear optic, such as non-linear optical crystals, like KDP, KD*P, KTP, LBO, BBO, and others. For a discussion of harmonic generation and solid state laser systems, see Koechner, "Solid-State Laser Engineering," 2d. Ed., Springer-Verlag, 1988, pp. 477-518, and particularly, pp. 514-518. See, also, Kleinman, et al., "Second Harmonic Generation of Light By Focused Laser Beams," Physical Review, Vol. 145, No. 1, May, 1966, pp. 338-346; and Boyd, et al., "Second-Harmonic Generation of Light with Double Refraction," Physical Review, Vol. 137, No. 4A, February, 1965, pp. A1305-A1320.
To obtain the laser sum frequencies or harmonics, the non-linear optic may either be placed within the resonant cavity or external to it. These two techniques are referred to, respectively, as intracavity harmonic generation and extracavity harmonic generation. A significant design choice exists between intracavity and extracavity harmonic generation due to the greater power efficiency provided by an intracavity design. See, for example, Chaoen, et al., "A stable 33W green laser obtained by intracavity frequency-doubling using a KTP crystal." Chinese Physics-Lasers, Vol. 15, No. 10, October, 1988, pp. 726-729. However, an intracavity design, in contrast to an extracavity design, presents substantial difficulties.
For example, an intracavity design exposes the non-linear optic to much higher radiation intensities than an extracavity design. In high power operation where the power density within the non-linear optic is increased, this can result in damage to the non-linear optic.
Intracavity designs also provide further difficulties in attempting to operate at high power and in multimode. For instance, as noted above, damage to the non-linear optic can result if the power density within the non-linear optic becomes too great. Since power density is in at least one aspect a function of spot size, controlling the spot size of the beam at the point of interaction with the non-linear crystal is critical. Thus, the design of the resonator for intracavity design must involve consideration of the spot size at the non-linear optic, in addition to analysis of resonator stability. In conventional lasers using Nd:YAG rods for the gain media, a thermal lensing effect occurs in the rod of Nd:YAG as pump power increases which can cause a decrease in the spot size in the non-linear optic. This results in an increase in the power density within the non-linear optic due to the additive effect on intensity of stronger thermal lensing (the YAG rod acts as a positive biconvex lens) and intracavity power increase. Therefore, limiting the effects of thermal lensing to protect the integrity of the non-linear optic is an important design problem.
One technique for controlling the spot size at the optic interface in such Nd:YAG rod systems is an optical relay described in detail in U.S. Pat. No. 4,907,235, invented by Kuizenga, issued Mar. 6, 1990, which is incorporated by reference as if fully set forth herein. Though the relay technique disclosed by Kuizenga is useful, it places limitations on the design of the laser system.
Another difficulty encountered in high power operation using an Nd:YAG rod as a gain media is the deterioration in the quality of the beam, especially where the beam includes multiple transverse modes. Deterioration of the beam is a significant problem in that it lowers the efficiency of the non-linear interaction, and therefore limits the amount of sum frequency or second harmonic power the laser system can generate. If the beam is multimode, the efficiency of the laser system drops even further. For a discussion of beam deterioration at high power and its impact on design limitations in laser systems, see Eimerl, "High Average Power Harmonic Generation," IEEE Journal of Quantum Electronics, Vol. QE-23, No. 5, May, 1987, pp. 575-592.
Laser systems for high power applications have been designed using a slab shaped gain medium, in which zigzag optical path is used. These slab laser systems are particularly useful in high power Nd:YAG embodiments, in which the zigzag optical path greatly minimizes the thermal focusing problem. However, such systems are highly multimode and do not provide a high quality output beam. For a discussion of the zigzag optical path in a slab laser system, see Koechner, "Solid State Laser Engineering," 2nd Ed., Springer-Verlag, 1988, pp. 391-397.
A laser system using a slab shaped gain media has been used for second harmonic generation with the non-linear crystal outside the resonant cavity is set forth in Boczar, "Second Harmonic Generation of Slab Lasers," SPIE, Vol. 736, 1987, pp. 60-64, with reference to Rapoport, et al., "Nd:YAG Slab Oscillators, Amplifiers, and Optical Phase Conjugation," SPIE, Vol. 736, pp. 65-71. However, the extracavity non-linear optic, as noted previously, does not present the design difficulties associated with intracavity designs. Further, the Boczar reference operates in single axial mode, which again does not present the problems associated with multimode operations, as disclosed by the present invention.
It is desirable therefore to provide intracavity frequency doubled or some frequency systems producing high powers in the output wavelength, and particularly high power greater than sustained 35 watts, in generically viable forms.